1. Field of the Invention
The present invention relates generally to an embedded passive device structure and manufacturing method thereof, and in particular, to an embedded passive device structure of higher device characteristic value (such as, for example, capacitance) and manufacturing method thereof.
2. The Prior Arts
Embedded passives are capacitors or resistors fabricated directly in the inner layers of a multi-layer PCB by means of etching or printing process in the formation of the inner layers during the manufacturing process of the multi-layer PCB. After the multi-layer PCB is laminated and manufactured, these embedded passives can replace the discrete passive components that would have to be soldered on the board surface of the multi-layer PCB. As a result, more board surface areas are available for layout and connection of active device components on the PCB.
The embedded, implanted or the buried technology is staffed by Ohmega-ply Company. By forming a nickel phosphorus alloy thin film layer on the matte side of the original copper-cladding on an inner-layer surface, resistive elements can be embedded and laminated to form the thin core. Using three etching steps and two photoresist layers, the required thin film “resistor” on a specific designated position can be fabricated. Because the above resistor is embedded inside the thin core, it is commonly known as a Buried Resistor (BR).
Thereafter, in 1992, a PCB manufacturer in the U.S.A named Zycon® adds in an extremely thin (2-4 mil) dielectric layer as the thin core at the upper layers inside the multilayer PCB, which is outside of the original Vcc/GND thin core, and uses the vast area of the parallel copper sheet/foil of the thin core for manufacturing an integrated capacitor, with the trade name of Buried Capacitor™ (BC). It has advantages such as interference reduction, offering more charge capacity, and providing steady voltage while operating under fundamental frequencies. Zycon® Company had applied for several patents relating to the BC (namely, the U.S. Pat. Nos. 5,079,069, 5,161,086, and 5,155,655).
Generally, in order to improve the capacitance of a capacitor under a stable voltage condition, other than the adoption of a high-k dielectric layer, using larger areas for the source electrode, and ground electrode is also adopted. The source electrodes and ground electrodes should be as close to each other as possible. However, if the embedded capacitor structure from the Zycon® Company is adopted because of the desire to adopt the ultra thin substrate, the manufacturing process becomes difficult, and the yield is lower at the same time. Inside this capacitor structure, the source electrode, the ground electrode, and the dielectric layer are sequentially stacked. In addition, for achieving higher capacitance, the ground electrode and the source electrode are already very much close to each other. Because of the extra force upon the upper layers of the source or ground electrodes during the lamination process, it is conceivable that some of the source electrodes or the ground electrodes may be embedded into the dielectric layer. Therefore, the source electrode and the ground electrode may contact with each other, thereby resulting in defective products. Although the conventional manufacturing technology was already dramatically improved by using the Zycon® method, under the general design criteria, the thickness of the dielectric layer at the minimum remains to be at 12 μm. As for adopting the high-k resin, the difficult part lies within the difficulty to control the final thickness during lamination after the printing steps. In addition, using the copper sheet having low profile is required to avoid the problem of producing cohesion at the same time. The reliability of the circuit board manufactured is questionable.